The work is focused on studies of the mechanism of antigen specific suppressor T cells (SC) in vitro. Antigen specific SC are induced in vitro with higher antigen concentrations than needed to induce helper cells in vitro. They are less dependent on their induction than are helper cells, but like helper cells, require T-T interaction for their generation. Their action is on helper pathways of the same antigenic specificity as SC, and there is no effect on thymus independent responses. Further studies demonstrated that cell to cell contact between SC and their target was not necessary but their function could be mediated by supernatants (SF) of metabolically active SC further cultured 24 H. in vitro after the initial 4 day induction period. SF share the antigen specificity of SC. SF do not affect thymus independent responses, but decreases thymus dependent IgM responses. Studies on the nature of SF are in progress, but decreases thymus dependent IgM responses. Studies on the nature of SF are in progress, and the experiments so far suggest that SF are heat labile and sensitive to proteolytic enzymes. Further studies are also under way for characterization of their target cell (which seems to be the actual helper cells passing through nylon wool) and genetics of their action.